Epoxy resin is one of the most widely used coatings for protecting steel due to its excellent chemical, corrosion resistance and versatility. Epoxy coatings can be formulated with a wide variety of starting resins such as Bisphenol A, Bisphenol F, Novolac Epoxy, and phenolic as well as a wide selection of curing agents such as polyamide amines, aliphatic amines, cycloaliphatic amines, aromatic amines and anhydrides. Also, epoxy coatings are available in liquid as well as in powder forms. An excellent summary of using epoxy protective coatings can be found in “Corrosion Prevention by Protective Coatings,” Chapter 5, Second Edition, by Charles Munger, published by the National Association of Corrosion Engineers (NACE).
The adhesion of epoxy resin to steel, while depending on the selection of curing agents, is generally satisfactory if the service temperature does not vary too much. However, it is well known in the coating industry that epoxy resins are prone to interfacial delamination which prevents it from many applications where there is a significant temperature cycling. Moreover, epoxy has low resistance to undercut corrosion. This means that if corrosion is initiated somewhere in the coating due to imperfection such as pin holes or holidays, the corrosion will quickly spread underneath the well coated areas where excellent adhesion between the epoxy and steel was originally achieved.
On the other hand, liquid ebonite has also been disclosed as monolithic protective coating to replace hard-rubber (or elastomer) sheet linings to protect steel. For prior art teachings on liquid ebonite coatings, readers are referred to U.S. Pat. Nos. 5,766,687 and 5,997,953, both by Rappoport, WO 0,006,639 by Figovsky, DE 3,740,181 by Petrovic et al., U.S. Pat. No. 6,482,894 by Chang et al., and U.S. patent application 2002/0002244 by Hoelter et al.
Ebonite coatings exhibit several outstanding properties including excellent adhesion to steel, resistance to delamination due to temperature cycling, and chemical resistance to a wide range of chemicals similar to their elastomeric sheet lining counterparts. A summary of the chemical resistance of elastomeric sheet linings can be found in “Coatings and Linings for Immersion Service, Revised Edition,” Chapter 9: Elastomeric Linings, published by NACE. Overall, ebonite coatings or elastomeric linings are not recommended for direct immersion in sulfuric acid with higher than 65% concentration. This restricts the applications of ebonite coatings to less severe conditions.
A liquid ebonite composition contains at least four major ingredients: liquid rubber, elemental sulfur, vulcanization accelerator, and vulcanization activator. There exist prior art compositions consisting of a blend of epoxy with liquid rubber alone, or of a blend of epoxy with sulfur or sulfur containing organic chemicals alone. For example, it is well known and widely practiced in the art to toughening an epoxy with a compatible liquid rubber. As described in U.S. Pat. No. 4,921,912, such composition provides improved physical properties such as peel adhesion, impact strength. However, there is no mention of corrosion resistance.
Blending an epoxy with sulfur or sulfur containing organic chemicals alone is also well known. For example, U.S. Pat. No. 4,389,501, disclosed that by adding elemental sulfur to an epoxy composition, the cure rate of the epoxy was accelerated. However, there was no mention of effect on adhesion or resistance to undercut corrosion. U.S. Pat. No. 4,153,740, disclosed an organo-sulfur compound such as thiuram, sulfenamide, or benzothizole in a resinous carrier including epoxy as a coating on the electrical wire for detecting wire overheating. All the organo-sulfur compounds disclosed therein can be used as a vulcanization accelerator. This prior art composition thus contained an epoxy and a vulcanization accelerator, but not liquid rubber, elemental sulfur, and vulcanization activator that are required in an ebonite composition.
These prior art teachings do not teach or suggest an epoxy composition that contains the four essential ingredients of a liquid ebonite composition, namely, liquid rubber, elemental sulfur, vulcanization accelerator, and vulcanization activator. Furthermore, none of the aforementioned prior art references teach or suggest a versatile, practical, and effective coating composition that has improved resistance to undercut corrosion.